154 Vertebrate Embryology 



5. The formation of the tubular heart and 

 of some of the blood vessels. 



6. The appearance of the Wolffian duct, or, 

 rather of a longitudinal rod of cells that will 

 later become hollow to form the duct. 



From the 36TH to the 48TH Hours. 



During the second half of the second day 

 the separation of the embryo from the yolk-sac 

 becomes much more plainly marked. This is 

 brought about by the formation of a tail-fold, 

 similar to the head-fold, and of lateral folds 

 (Fig. 38), so that by the end of this period, the 

 whole outline is distinct, from head to tail. 



The brain. — During ihis period the neural 

 canal becomes entirely closed, even the sinus 

 rhomboidalis being fused The constrictions 

 that mark off the anterior end of the neural 

 canal into what we have called the fore-, mid-, 

 and hind-brain become more evident, and be- 

 fore the close of the day the fore-brain begins 

 to grow forwards as an unpaired vesicle which is 

 the first indication of the cerebral hemispheres. 

 The walls of the brain lie close under the ecto- 

 blast, but between the two is seen, in sections, 

 a small amount of mesoblast which will form 

 the skull. 



